In the past twenty years research activity has increased dramatically in the field of conductive polymers. The reason was the discovery that conjugated polymers can behave as metallic conductors and semiconductors. These polymers did not gain commercial significance because they were not stable. In the last five years work has concentrated on increasing the environmental stability and processability of these polymers. See, M. C. Gallazi et. al. “Regiodefined Substituuted Poly(2,5-thienylene)s, J. Poly. Sci. Part A: Polymer Chemistry, Vol. 31, 3339-3349 (1993); G. Zotti et. al. “Novel. Highly Conducting, and Soluble Polymers from Anodic Coupling of Alkyl-substituted Cyclopentadithiophene Monomers”, Macromolecules 1994, 27, 1938-1942; and K. J. Ihn et. al. “Whiskers of Poly(3-alkylthiophene)s, J. Poly. Sci. Part B: Polymer Physics, Vol. 31, 735-742, (1993). In addition, it has been reported that the addition of side chains have rendered the various polymers more soluble and have therefore also stabilized, to some extent, their structure.
More recently, various papers have alluded to the advancement in semi-conducting polymer technology that are used as charge separators to produce photo-induced electron transfer. See, e.g., J. H. Burroughes et. al. “New Semiconductor Device Physics in Polymer Diodes and Transistors”, Nature, Vol. 335, 8 Sep. 1998; R. N. Marks et. al. “The Photovoltaic Response in Poly(p-phenylenevinylene) Thin-Film Devices” J. Phys.: Condens. Matter 6 (1994); C. W. Tang Appl. Phys. Lett., Vol. 48, No. 2, 13 Jan. 1986; J. H. Burroughes et. al. Nature Vol. 347, 11 Oct. 1990 pp. 539-541; N. C. Greenham et. al. Chem Phys Letters 241 (1995) 89-86; and G. Yu et. al. Appl Phys lett. 64 (25)20 Jun. 1994. These new materials provide “[a] molecular approach to high efficiency photovoltaic conversion” G. Yu et. al. The efficiency with which these polymers convert photons to electrons is near 100%. However, the overall efficiency of the cell is hindered by inefficient collection of the carriers. See, G. Yu et al. J. Appl. Phys. 78 (7), 1 Oct. 1995. N. C. Greenham et. al. reported “[t]he problem of transport of carriers to the electrodes without recombination is a more difficult one to solve, since it requires that once the electrons and holes are separated onto different materials, each carrier type has a path way to the appropriate electrode without needing to pass through a region of the other material”. See, Phy. Rev. B, Vol. 54, No. 24, 1996, pp. 17628-17637. This problem appears to be the roadblock to continuing progress in this field.
G. Yu et. al. (J. Appl. Phys, 78 (7), October 1995) and J. J. M. Halls et. al. (Nature, Vol. 376 (1995) 4510) suggested the problem can be alleviated by phase separation of the two charge carriers, thereby causing the photoinduced reaction at the donor/acceptor (D/A) interface to occur at the boundary between the two phases while allowing the separate carriers to migrate through their own phase (see FIG. 1). This solution offered some increase in efficiency (˜2%), however, the disorder in the phase separated regions did not allow easy collection of the carriers.
A. J. Heegar (TRIP Vol. 3, No. 2, February 1995) attempted to increase the order in the phase separated blends by creating a network of one polymer in the other. The increase in efficiency was only marginal (˜2.3%). The closest attempt to create the ideal structure was accomplished by B. O'Reagn et. al. (Nature, Vol. 353 (1991) 737) and U.S. Pat. No. 5,084,365. They accomplished it by using a mixture of dyes and nanometer sized titania particles. The resultant cell gave an energy conversion efficiency of 12%. The reason for this significant increase was due to the large surface area afforded by the nano particles in close proximity to the charge transfer couple material. In order to get this, however, they had to use liquid electrolyte that seeped into the “nooks and crannies” of the porous anode to establish an electric connection with the light active sites and harvest the holes that were created by them. The porous nature of the anode created the large D/A surface area while the liquid electrolyte acted as the hole collector in this case. In addition collection was done through an electrolyte via ion charge transfer rather than actual hole transport. The above work demonstrated that with an efficient collection scheme the overall energy conversion efficiency could be dramatically improved. The above cell however, is not conducive to large area roll to roll manufacturing, since it used liquid as one of the components in the cell.
Since then many researchers have tried to replace the liquid electrolyte with various solid electrolytes and other solid hole conductor materials with little success (a few % efficiency). See, e.g., Kei Murakoshi et. al. Chemistry letters, 1997 p. 471-472; A. C. Arango et. al. Mat. Res. Soc. Symp. Proc. Vol. 561 pp. 149-153; A. C. Arango et. al. Applied Phys. Letters Vo. 74, No. 12 pp. 1698-1700; K. Yoshino et. al. IEEE Trans. Elec. Dev 44 (8), p. 1315-1323 (1997); and R. N. Marks et. al. J. Phys Condens. Matter 6, p. 1379-94 (1994); and T. J. Savenje et. al. Chem Phys. Lett. 290, p. 297-303 (1998). The conclusions always returns to the same basic problem: to avoid recombination losses the layers need to be made very thin which in turn diminishes light absorption and hence charge generation. Alternatively, if the two materials are intermixed charge generation is high, but, collection is severely repressed due to a high recombination rate. Charge collection remains the key challenge for high efficiency and hence the commercialization of polymeric based solar cells.
To optimize the efficiency, and avoid the above problems of the prior art, an object of the present invention is to develop a photovoltaic cell wherein one electrode is connected to all the separate donor phases to collect the holes and another electrode is connected to all the individual acceptor regions to collect the electrons, thereby resulting in a high efficiency photovoltaic cell.
More specifically, it is an object of the present invention to develop the above referenced photovoltaic cell wherein rods of one phase (the electron accepting anode) are placed inside the matrix of the other phase (i.e., the electron donating cathode). Hence charge separation would be made to occur at the interface between the rods and the matrix while charge transport would take place through the rod anodes for one carrier and through the matrix cathode for the other.
Finally, in accordance with the objectives of the present invention, a means of making an electrical contact to all the electrical accepting anode rods in the matrix needs to be established. Such method then provides the means to allow charge collection into a single point i.e. to the electrode.
The present invention also has as its object the development of a structure to be used as an efficient means of collecting free electrons created by charge separation from photon excitation of a donor/acceptor pair. This configuration will then allow electron collection through the central point collector in the acceptor phase and hole collection via the electrical contact of the donor phase with a metal conductor layer.